The invention relates to an illumination system for illuminating a display device, which illumination system is provided with a light-emitting panel comprising
a front wall, a rear wall situated opposite said front wall, and edge areas between the front wall and the rear wall,
at least one of the edge areas of the panel being light-transmitting,
at least one light source being associated with the light-transmitting edge area, and
in operation, light originating from the light source being incident on the light-transmitting edge area and spreading in the panel.
The invention also relates to a display device comprising said illumination system.
Such illumination systems are known per se and are alternatively referred to as edge-lighting systems. They are used, inter alia, as backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as a backlight for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones.
Said display devices generally include a substrate provided with a regular pattern of pixels, which are each driven by at least one electrode. In order to reproduce an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device, the display device uses a control circuit. In an LCD device, the light originating from the backlight is modulated by means of a switch or a modulator, while applying various types of liquid crystal effects. In addition, the display may be based on electrophoretic or electromechanical effects.
In the illumination systems mentioned in the opening, paragraph, customarily a tubular low-pressure mercury-vapor discharge lamp, for example one or more compact fluorescent lamps, is used as the light source, the light which, in operation, is emitted by the light source being coupled into the light-emitting panel which serves as an optical waveguide. This optical waveguide generally forms a comparatively thin and flat panel which is made, for example, of a synthetic resin or glass, light being transported through the optical waveguide under the influence of (total) internal reflection.
Such an illumination system may also be provided with an alternative light source in the form of a plurality of optoelectronic elements, also referred to as electro-optic elements, such as electroluminescent elements, for example light-emitting diodes (LEDs). These light sources are generally provided in the proximity of or contiguous to a light-transmitting edge area of the light-emitting panel, so that, in operation, light originating from the light source is incident on the light-transmitting edge area and spreads in the panel.
EP-A 446 324 discloses an illumination system which causes an LCD panel to be illuminated at different types of ambient light. For the light source use is made of incandescent lamps which couple light into a light-emitting panel, also referred to as light pipe. In the light-emitting panel, multiple reflections of light bring about a light distribution to illuminate the LCD panel.
An illumination system of the above-mentioned type has the disadvantage that the light distribution in the light-emitting panel, particularly in the proximity of the light source, is insufficiently uniform. As a result, the illumination uniformity of the display device is insufficient.
It is an object of the invention to completely or partly overcome the above-mentioned drawback. The invention more particularly aims at providing an illumination system of the type mentioned in the opening paragraph, wherein the uniformity of the light distribution of the illumination system and hence the uniformity with which the display device is illuminated are improved.
In accordance with the invention, this object is achieved in that,
the illumination system comprises at least two light-emitting panels, which panels are arranged at least substantially parallel to each other, and
sub-surfaces of the rear walls of each of the light-emitting panels are provided with means for coupling light out of the panel,
the sub-surfaces projecting, in operation, light on regions of a projection area extending parallel to the light-emitting panels, and
the projected regions in the projection area at least being contiguous.
The use of a plurality of light-emitting panels in the illumination system in accordance with the invention has the advantage that the sub-surfaces of each of the rear walls of the light-emitting panels co-operate with each other so as to uniformly illuminate the display device. Both the choice of the sub-surfaces and the distribution of the means for coupling out light over the sub-surfaces are such that the light emitted, in operation, by each of the sub-surfaces uniformly illuminates the (imaginary) projection area. As the projection area corresponds to the display device surface to be illuminated, a uniformly illuminated projection area leads to a uniform illumination of the (image) display device.
By virtue of the measure in accordance with the invention, the uniformity of the distribution of the light emitted by the illumination system is improved. As a result, a more uniform illumination of the (image) display device is obtained.
A further advantage of the application of multiple light panels resides in that the light output and the light distribution of each of the panels can be influenced individually.
In order to save space, the light panels are preferably arranged one behind another.
The arrangement of the sub-surfaces with respect to the light source influences the way in which light issues from the light-emitting panel. A preferred embodiment of the illumination system in accordance with the invention is characterized in that the sub-surfaces are situated at a distance from the light-transmitting edge areas.
Particularly in the proximity of the light source, the light distribution of the known light-emitting panel is insufficiently uniform. If, in accordance with the invention, the sub-surfaces, which couple light out of the panel, are chosen so as to be not adjacent or near the light source associated with the relevant light-emitting panel, the,n the influence of the light source on the light coupled out by the panel is reduced.
The shape of the sub-surfaces and their position with respect to each other, in combination with the way in which the means for coupling out light are distributed over the sub-surfaces, influence the way in which light issues from the light-emitting panels. A favorable embodiment of the illumination system is characterized in accordance with the invention in that each of the sub-surfaces comprises a single surface covering approximately half the rear wall of the light-emitting panel, the means for coupling, out light being provided such that the degree to which light is coupled out is substantially constant over the sub-surface.
In this preferred embodiment, the illumination system comprises two light-emitting panels, each of the light-emitting panels being responsible for approximately half the light emitted by the illumination system. The means for coupling out light are uniformly distributed over said sub-surfaces in such a manner that the light emitted by the sub-surfaces very uniformly illuminates the corresponding part of the imaginary projection area. A uniformly illuminated display device is obtained by arranging the sub-surfaces preferably on a part of the rear wall which is situated at a (maximum) distance from the light-transmitting edge-area.
It is particularly favorable if the light-transmitting edge areas are alternately situated at opposite sides of the panels. As a result, the sub-surface of one light-emitting panel illuminates one half of the display device, for example the lower half, while the sub-surface of the other light-emitting panel illuminates the other half of the display device, for example the upper half. This has the advantage that the two halves of the display device may have different illumination levels. A further advantage thereof is that the display screen of the display device is suitable for spatial sequential operation. In an alternative embodiment, a ridge is formed in one of the sub-surfaces or in both sub-surfaces, thereby effectively precluding the mixing of light between the two halves of the display device.
In an alternative embodiment, the illumination system comprises three light-emitting panels, each of which couples out approximately one third of the light.
In addition to the above-described uniform distribution of the means for coupling out light over the sub-surfaces, it is advantageous, in alternative embodiments, to distribute the means non-uniformly over the sub-surface. Also a suitably chosen inhomogeneous distribution of the means for coupling out light enables the light emitted by the various sub-surfaces to very uniformly illuminate the imaginary projection area. For this purpose, a preferred, alternative embodiment of the illumination system in accordance with the invention is characterized in that the sub-surfaces extend over a substantial part of the rear wall of the light-emitting panels, the means for coupling out light being arranged such that the degree to which light is coupled out changes as a function of the distance to the light-transmitting edge area.
If the means for coupling out light are uniformly distributed over the sub-surfaces, particularly the interfaces in the projection area between light originating from the various sub-surfaces are sensitive to misalignment of the light-emitting panels. The inventors have recognized that if the means for coupling out light are not uniformly distributed over the sub-surfaces, the degree to which light is coupled out changing as a function of the distance to the light-transmitting edge area, the light distribution over the imaginary projection area as a result of coupling out light by the light-emitting panels becomes less sensitive to obliqueness of the light-emitting panels or to a sub-optimal alignment of the light-emitting panels.
Preferably, the means are distributed over the entire rear wall of each of the light-emitting panels, the degree to which light is coupled out being comparatively small in the proximity of the light-transmitting edge areas, and comparatively large in the proximity of the opposite edge area. In this embodiment, each one of the light-emitting panels contributes to the illumination of at least substantially the entire surface of the projection area, but the contribution to the illumination of the light-emitting panel in the proximity of the light-transmitting edge areas is so small that effects of the light source on the light coupled out by the panel are negligibly small. In this embodiment, the degree to which each of the light-emitting panels contributes to the illumination of an imaginary point of the projection area depends substantially upon the position of said imaginary point in the projection area.
In accordance with a particularly suitable distribution, the means for coupling out light are arranged such that the degree to which light is coupled out changes linearly as a function of the distance to the light-transmitting edge area. In an alternative, favorable embodiment, the means are arranged such that the coupling out of light changes as a square of a sine or a cosine.
In an alternative embodiment of the illumination system in accordance with the invention, one of the light-emitting panels does not extend over the whole length of the projection area. This is suitable, in particular, if there is a lack of space on one of the two sides of the illumination system. This is the case, for example, if the connection wires for electrodes at edges of the LCD display device are diverted round the illumination system. The connection wires are customarily situated at two successive sides of the LCD display device. As a result, there is ample room for the light source of the illumination system at two of the sides of the display device, while there is comparatively less pace at the other, opposite sides. By shortening one of the light-emitting panels, more space is created at the latter side of the display device.
Preferably, the light source comprises at least one light-emitting diode having a luminous flux of at least 5 lm. LEDs having such a high output are also referred to as LED power packages. The use of these high-efficiency, high-output LEDs has the specific advantage that, at a desired, comparatively high light output, the number of LEDs may be comparatively small. This has a positive effect on the compactness and efficiency of the illumination system to be manufactured. Further advantages of the use of LEDs are a comparatively very long service life, comparatively low energy costs and comparatively low maintenance costs of an illumination system comprising LEDs.
The use of LEDs has the further advantage that dynamic illumination possibilities are obtained. For this purpose, a preferred embodiment of the illumination system in accordance with the invention is characterized in that a sensor for measuring the optical properties of the light emitted, in operation, by the light source is situated at the edge area of the light-emitting panels that is situated opposite the light-transmitting edge surface. If different types of LEDs are combined and/or LEDs of different colors are employed, colors can be mixed in a desirable manner, for example, to make white light of the desired color temperature. In addition, color changes can be brought about irrespective of the condition of the display device. The sensor can alternatively be provided on a different (edge) area of the light-emitting panel.
In a further preferred embodiment, the illumination system comprises control electronics for changing the luminous flux of the light source. Suitable control electronics enable the desired illumination effects to be obtained and the uniformity of the coupled-out light to be improved. In addition, a suitable combination of LEDs enables white light to be obtained, and a desired color temperature can be adjusted by means of control electronics.
In a preferred embodiment of the illumination system, the light source comprises at least two light-emitting diodes having different light-emission wavelengths. Such diodes emit light in a predetermined wavelength range. The LEDs comprise preferably the combinations of red, green and blue LEDs which are known per se, or combinations of red, green, blue and amber LEDs.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.